Non-volatile memories, such as EEPROM or flash memories, are widely used in portable devices including devices lacking a mass data storage device and a fixed source of power, such as cellular phones, handheld personal computers (PCs), portable music players and digital cameras. These memories can be erased and have new data written or “programmed” into their memory cells for long term storage. Flash memories are also commonly used to hold boot code in PC's, as well as used in servers, networks, and set top boxes.
A floating gate memory cell employs a floating conductive gate, within a transistor structure, but unconnected and positioned over a channel region in a semiconductor substrate, between source and drain regions. A control gate is provided over the floating gate. The threshold voltage characteristic of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage (threshold) that must be applied to the control gate before the transistor is turned “on” to permit conduction between its source and drain regions is controlled by the level of charge on the floating gate.
A floating gate memory cell can be programmed to any threshold voltage level within a threshold voltage window because the floating gate can retain a range of charge. The size of the threshold voltage window, limited to the minimum and maximum threshold levels of the device, depends on the characteristics of the device, operating conditions and the device's history. Each distinct, resolvable threshold voltage level within the window may, in principle, be used to designate a definite memory state of the cell.
In practice, the memory state of a floating gate memory cell is usually read by a sense amplifier circuit by sensing the conduction current across the source and drain electrodes of the cell when a reference voltage is applied to the control gate. Thus, for each given charge on the floating gate of a cell, a corresponding threshold voltage may be detected, or equivalently, a corresponding conduction current with respect to a reference control gate voltage may be detected. Likewise, the range of charge programmable onto the floating gate defines a corresponding threshold voltage window or a corresponding conduction current window.
A conventional sense amplifier circuit includes a differential amplifier circuit that generally senses a voltage differential between the voltage appearing on a column line connected to a reference cell and the voltage appearing on a column line connected to an addressed memory cell. It also drives a sense output signal (that is coupled to the data output pins of the flash memory device) based upon the sensed voltage differential.
Conventional flash memory devices include a reference current generator that generates a reference current for use by the sense amplifier circuits. A current mirror circuit in the flash memory device mirrors the reference current and applies a single mirrored reference current to all of the sense amplifiers. A startup circuit is utilized in some existing flash memory devices in order to provide a fast settling time of the reference node appearing at the input of the sense amplifiers. An existing startup circuit includes first and second discharge current stages, with each discharge current stage discharging the charge appearing at the reference node input of the sense amplifiers based upon a bandgap reference current. Each discharge current stage utilizes feedback to gradually decrease the rate of discharge by the discharge current stage so that the discharge current stages are disabled by the time the voltage appearing at the reference node input of the sense amplifiers reaches the desired voltage level.
The known reference current generator, startup circuits and sense amplifiers, however, have shortcomings. For instance, the use of the reference current generator and corresponding current mirror circuit limits the number of sense amplifiers that may be utilized at one time. There is also a relatively slow settling time of the reference voltage due to the large capacitive loading on the reference current generator/mirror circuit when a large number of sense amplifiers are used. There is also the shortcoming that both the memory cell current and the flash cell current vary over time due to injected noise, and the accuracy of conventional sensing techniques will be affected by many factors including the amount of noise affecting the currents, and the sensitivity, gain, offset, and noise immunity of the sense amplifier.
Accordingly, there is a need and desire for an improved sensing method and apparatus and to more effectively and accurately provide reference current levels to sense amplifiers in a nonvolatile memory device, such as a flash memory device.